improve description about enumerator in for-comprehensions

[laglangyue]

Student of Scala may have some doubts when learning for-enumerators, and may not understand the execution order of if enumerator. They may think that if is similar to match grammer, they think if i + j == v will execute before j <-....
look at this example

    val a = Array(1, 2, 3)
    val value = for
      i <- 0 to 5;
      j = a(i) if i % 2 == 0
     yield (j)

Actually, it should be

    val a = Array(1, 2, 3)
    val value = for
      i <- 0 to 5 if i % 2 == 0
      j = a(i) 
     yield (j)

[bishabosha]

@scala/docs-zh

[laglangyue]

sorry，I check this PR again，My friend doesn't read the documentation very well.
This section has explained very clearly how if works in for-comprehensions.